Pierce nuts have been used in mass production applications for many years. In the automotive industry, they are used to allow the mounting of components in metal sheets and panels. In general, to attach these components, threaded screws or bolts are used, and the joint can be tightened by means of a torque wrench.
Previously, there was a general tendency in the automotive industry to use thin metal sheets and panels to reduce the weight of the vehicles. Thus, the focus was on the development of pierce nuts designed to provide high torque resistance and pull-through resistance, also when used with very thin sheets.
The latest trend in the automotive industry is toward an extended use of high-strength sheet metal. By optimizing each individual component of a vehicle with respect to its intended function, it is possible to reduce the weight and the impact on the environment and also to improve vehicle safety. Thus it is necessary to have pierce nuts designed to provide even higher torque resistance and pull-through resistance, requirements that cannot be satisfied by existing nuts. Other affected parameters whose importance increases are punching force, tool damage and nut centering; as the strength of the punched metal sheet increases, greater punching force is required, which increases the load on the punching tools as well as the risk of alignment errors occurring between the pierce nut and the punching die, thereby increasing the risk of tool damage.
Pierce nuts should be attached to the metal sheet in such manner that they resist both pull-through forces and torque.
U.S. Pat. No. 5,340,251 proposes a pierce nut for attachment to a plastically deformable metal sheet, comprising an engagement side for engaging the metal sheet and a pilot extending from the engagement side in the direction away from the engagement side, which pilot has an end surface that faces away from the engagement side, and a first undercut portion facing away from the pilot and having a wall surface surrounding the end face. Moreover, the pierce nut has a second undercut portion oriented towards the pilot, and a groove located between the two undercut portions. When using the pierce nut disclosed in U.S. Pat. No. 5,340,251, the sheet is plastically deformed and pressed into the groove and beneath the two undercut portions, thereby to achieve a form-fit interlock between the nut and metal sheet. The pierce nut disclosed in U.S. Pat. No. 5,340,251 requires substantial deformation of the metal sheet to push it under the undercut portions, which in connection with high-strength sheet metal increases the stress on the pilot of the pierce nut, necessitates great punching force and, thus, increases wear also on the punching die. In addition, when using the pierce nut according to U.S. Pat. No. 5,340,251, a thin section is obtained between the punching die and the engagement side of the nut, which section is susceptible to fatigue failure.
In European Patent No. 0 554 599 B1, a pierce nut for attachment to a plastically deformable metal sheet is disclosed which comprises an engagement side for engaging the metal sheet and a pilot extending from the engagement side in the direction away from the engagement side, which pilot has an end surface that is facing away from the engagement side and a wall surface surrounding the end surface. The pierce nut further has an undercut portion oriented towards the pilot, and a groove located between the pilot and the undercut portion. When using the pierce nut according to EP 0 554 559 B1 for punching high-strength sheet metal, an insufficient amount of sheet metal is pushed into the groove and beneath the undercut portion, which results in a joint with low pull-through strength. In addition, greater punching force is required to deform the metal sheet, which may cause damage to the punching die. Furthermore, the pierce nut disclosed in EP 0 554 599 B1 is susceptible to alignment errors between the pierce nut and the punching die, which may result in the pierce nut threads being damaged or their mechanical properties becoming irregular.
German Patent No. DE 2004 017 866 A1 discloses a pierce nut for attachment to a plastically deformable metal sheet.
Further examples of pierce nuts are shown in DE 103 48 851 A1; US 2005/0186046 A1; U.S. Pat. No. 6,986,629 B2; US 2005/0147481 A1 and JP 64-58807.
There is a need for an improved pierce nut, which eliminates or reduces the problems discussed above.